1. Field of the Invention
The invention relates to underwater acoustic projectors and is directed more particularly to a projector of the type commonly referred to as a "push-pull" acoustic projector.
2. Description of the Prior Art
Acoustic projectors typically are used to produce high-power low-frequency sounds in the ocean or other water bodies. It generally is desired to be able to generate hundreds of watts of omnidirectional acoustic power with a device that can be deployed from an aircraft, surface vessel, or submarine.
Examples of such devices are shown in U.S. Pat. Nos. 4,706,230 to Inoue et al., 4,964,106 to Bromfield, 4,970,706 to Tocquet et al., and 5,345,428 to Arnold et al. The Inoue et al. patent illustrates an underwater low-frequency ultrasonic wave transmitter in which non-active columnar members are disposed on both sides of an active columnar member consisting of a piezoelectric ceramic material or a magnetically strainable material. Levers are connected to the active and non-active columnar members via first and second hinges. Convex shells are connected to the levers via third hinges. The displacement of the active columnar member is enlarged via the lever action, thereby enabling a miniaturized ultrasonic wave transmitter having high power capability.
The Bromfield patent relates to a flextensional sonar transducer assembly which includes a stack of piezoelectric elements disposed along a linear axis, a plurality of electrodes disposed between the elements, end pieces disposed at each end of the piezoelectric stack, with the end pieces having outwardly facing, generally arcuate surfaces, a compression band formed into a loop to encircle the stack and end pieces, a flexural shell disposed to circumscribe the compression band to present a generally elliptical side cross-section, with the major axis thereof being generally coincident with the linear axis of the stack, and with the shell being reactively coupled to the arcuate end portions of the compression band, and rod members positioned between the compression band and the arcuate end portions of the band of material for maintaining driving contact between the shell and the end pieces, and for adjusting the prestress compression pressure in the stack.
The Tocquet et al. patent relates to a flextensor transducer which includes at least one pillar of piezoelectric cells placed within an impervious shell. Each pillar is supported solely by a first end on the shell and is compressed on the shell by a counter mass applied to its second end.
The Arnold et al. illustrates a low frequency flextensional transducer for underwater use which comprises a number of spaced piezoelectric element stacks between opposed inserts. A Kevlar compression band is wound around the stacks and inserts and then partly elliptical plaster formers are attached. A filament wound elliptical flexural shell is then wound around the assembly while controlling the tension so as to provide the required prestress on the piezoelectric stacks when cured. After curing the plastic formers are removed. End plates are attached to the elliptical shell to complete the transducer. The shell has a compression bonded layer of neoprene applied, including a peripheral serrated lip seal to seal against the end plate while permitting flexing of the shell.
In order to create high-power acoustic tones in water at low frequencies, it has been found that a device must be able to produce large volume displacements. The volume displacement is the integral of the normal displacement of a radiating area, taken over that area. Therefore, an acoustic projector must have a large radiating area, or a large displacement, or both. It is beneficial to have a projector with an output that varies linearly with the projector input.
The term "push-pull" as used herein and in the appended claims refers to a mode of operation of a pair of electrostrictive annular rings, in which the rings are in abutting relation to one and the other faces of a plate. The rings operate in unison, but oppositely, such that one ring "pushes" the plate, while the other ring simultaneously "pulls" the plate. Such an arrangement is commonly referred to as a "push-pull" projector, or transducer. The mode of excitation produced thereby is termed "flexural excitation" which is the direct result of bending moment.
U.S. Pat. No. 3,725,856 to Chevenak illustrates a push-pull transducer. In the Chevenak device, the driver plate is not attached to the walls of a hollow cylinder formed by a plurality of walls.
In recent years, consideration has turned to electrostrictive rings of lead magnesium niobate (PMN) and lead magnesium niobate-lead titanate (PMNPT) ceramics. For example, U.S. Pat. No. 5,359,252 to Swift et al. discloses an actuator in which a stack of lead magnesium niobate crystals are free to expand longitudinally within a cylindrical casing to act on a piston. U.S. Pat. No. 5,493,165 to Smith et al. discloses a driver for electrostrictive actuators in which rings of lead magnesium niobate are interleaved with electrode rings to form a stack.
Lead magnesium niobate and lead magnesium niobate-lead titanate have recently gained wide interest in the underwater acoustics transduction community due to the propensity of these materials to exhibit large strains at relatively modest electric field levels. One difficulty with these materials, however, is that the observed strains exhibit a nonlinear response to the applied drive voltage. That is, harmonics of the drive frequency appear in the response of the material. However, the nonlinearity of both PMN and PMNPT is known to exhibit a primarily quadratic response of the strain to the applied drive. See K. M. Rittenmeyer, "Electrostrictive Ceramics for Underwater Transducer Applications," J. Acoust. Soc. Am. 95, pp. 849-856 (1994). This quadratic behavior has been shown to permit the application of a revised Hunt electrostatic transducer model (F. V. Hunt, Electroacoustics, John Wiley & Sons, New York, 1954, pp. 176-177) to understanding the behavior of a PMN or PMNPT-based transducer. See J. C. Piquette, "A Fully Mechanical Transducer Model With Application to Generalizing the Non-Linear Hunt Electrostatic Transducer for Harmonic and Transient Suppression," J. Acoust. Soc. Am. 98, pp. 422-430 (1995). Since PMN and PMNPT behave in a manner similar to a Hunt electrostatic transducer, concepts applicable to linearizing an electrostatic transducer are also applicable to developing a linear underwater projector which utilizes either a PMN and/or PMNPT active element. Hence, applicant has recognized the concept of push-pull electrostatic loudspeaker is applicable to the development of a push-pull underwater projector using PMN and/or PMNPT. It has also been recognized that since the properties of PMN are sensitive to the operating environmental conditions, it is also important to correct the operation of a PMN-based projector for variations in environmental conditions.
The operation of an electrostatic loudspeaker is based on the push-pull principle. This principle produces cancellation of nonlinear responses that arise from a quadratic nonlinearity. The push-pull electrostatic loudspeaker operates in a mode which attempts to maintain a constant charge on a pair of balanced moving plate capacitors which share a common moving plate. It has been recognized by applicant that an underwater projector using a PMN and/or PMNPT active element can also be designed to take advantage of the push-pull concept, and hence can also produce a linearized output in a manner similar to an electrostatic loudspeaker.